1. Field of the Invention
The present invention relates to lamp electrodes adapted to deliver mercury and to methods for delivering mercury to a lamp.
2. Description of Related Art
Conventional gaseous discharge lamps employ a metallic electrode in the form of a tubular shell that is open at the distal end and closed at the proximal end. The proximal end of the shell is supported at the hairpin turn of supporting electrical leads, whose two legs are embedded in a pinch seal made in a short tubular glass body. A working discharge lamp is typically fabricated in the field by fusing the short glass tubes of the electrode assemblies to both ends of a longer glass tube that was internally coated with a phosphorescent material.
An evacuation tube can be included as part of one of the electrode assemblies in order to communicate with the interior of the discharge lamp. Before loading fill gases into the lamp, the electrode shells are bombarded with charged particles in the usual fashion in a partial vacuum. Thereafter, working with the evacuation tube, a vacuum is pulled before loading an inert gas and tipping off the evacuation tube.
For the sake of efficiency a discharge lamp will typically have a dose of mercury. During normal operation the mercury atoms (existing as a vapor in the lamp) are stimulated by an electrical discharge between the two electrodes and emit UV radiation when returning to a lower energy state. This UV radiation will stimulate the phosphorescent coating on the inside of the long glass tube to produce visible light.
While mercury has its benefits it is also a toxic substance and care must be taken to avoid injury and to ensure accurate dosing. It is especially desirable to avoid handling mercury in the field or relying on the measurement skill of field personnel to ensure correct mercury dosing.
In addition, care must be taken to contain the mercury to avoid an accidental release into the environment, which can adversely affect water quality, fish, and wildlife. It has been determined that containment and safety is enhanced if the mercury is confined to a small container until the discharge lamp is fully sealed, at which point the mercury container can be opened to release the mercury dose.
Also, care must be taken to avoid a premature discharge of mercury before the lamp is completely sealed. The mercury container can prematurely open when exposed to the high temperatures that are often experienced during the manufacture of electrodes and during the fabrication of a working discharge lamp in the field. For example, during the manufacture of electrodes one end of a relatively short glass tube is melted to form a pinch seal on the leads. During fabrication in the field, before the lamp is fully sealed, the electrode shell are “bombarded” with a high current and heated glowing red
When release of the mercury is desired, such release ought to be reliable without risking damage to the finished lamp. Furthermore, the mercury should be released in a location and in a direction to ensure the mercury will be available while avoiding condensation that may stain lamp components and degrade their appearance.
Miniature movement-detection switches have employed a small container sealed with a header. A drop of liquid mercury in the container can make a connection between the metal container and a lead projecting into the container through an insulating glass feedthrough in the header. See the miniature switches offered by Comus International; Clifton, N.J.